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Different substances respond to input of energy in different ways. We say that they have a particular 'capacity' for the energy applied. Here we examine the reasons for this and its consequences. Syllabus referenceReactivity 1.1.4 - The standard enthalpy change for a chemical reaction, ΔH⦵, refers to the heat transferred at constant pressure under standard conditions and states. It can be determined from the change in temperature of a pure substance.
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Heat capacity
All systems are made up of components and substances that have different structures and different fundamental particles. Consequently they have differing abilities to absorb heat energy, and produce different temperature changes on absorption of energy. This is called the heat capacity of the system. It literally means the capacity, or absorbing ability that a substance, or system, has for heat energy.
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Example: A calorimeter can absorb 100kJ of energy with a resultant increase in temperature of 1ºC. This means that whenever the calorimeter absorbs 100kJ its temperature increases by 1ºC. In other words, its heat capacity is 100 kJ ºC-1. If it were to absorb 200kJ then its temperature would increase by 2ºC, if it were to absorb 300kJ it would increase in temperature by 3ºC, etc etc. |
If the heat capacity of a system is known, or found by, calibration (using heating coils, or known chemical reactions), this can then be used to measure the energy released by other chemical reactions.
Specific heat capacity
The amount of energy that a given mass of substance (either 1 g or 1 kg) can absorb that produces a 1ºC increase in temperature is called it's specific heat capacity. This is sometimes given the symbol 'shc' or simply 'c' .
The specific heat capacity of water is 4.18 kJ kg-1 ºC-1. This means that when 1 kg of water absorbs 4.18 kJ of energy its temperature will increase by 1ºC.
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Example: Calculate the amount of energy needed to raise the temperature of a bathtub containing 100kg of water by 20ºC. (ignore the energy required to heat the material of the bathtub itself.)
Energy needed = 100kg x 4.18 kJ kg-1 ºC-1 x 20ºC Energy needed = 100 x 4.18 x 20 = 8360 kJ |
Water has a relatively high specific heat capacity. Metals, such as copper, have much lower specific heat capacities, so the temperature rise is greater for the same input of energy. The following table shows the specific heat capacity of some substances and the effect in terms of temperature change when 100g of each substance is provided with 1 kJ of energy.
| Material | specific heat capacity kJ kg-1 ºC-1 |
Temperature change when 100g receives 1 kJ |
| Aluminium | 0.897 | 11ºC |
| Copper | 0.385 | 26ºC |
| Gold | 0.129 | 77ºC |
| Iron | 0.450 | 22ºC |
| Water | 4.18 | 2.4ºC |
The above table illustrates the large capacity that water has to absorb energy when compared to metals.
Water having a low relative molecular mass has more particles per unit mass able to absorb energy, while maintaining the average energy at a lower value. The temperature of any substance is proportional to the average energy of the particles in the material.
